The present disclosure relates to downhole electromagnetic induction tools and, more particularly, to apparatus and methods for electrically insulating an electromagnetic (“EM”) induction tool to minimize azimuthal current induced by a transmitter on tubulars of the induction tool. As disclosed herein, the term “electromagnetic induction tool” may denote any electromagnetic tool which works at least in part based on induction principles. The term “electromagnetic induction tool” is not intended to limit the application to subterranean formation resistivity measurement and specifically includes ranging applications, where a distance and/or direction to a second wellbore may be calculated.
In well operations, it may be desirable to survey the formation using a downhole tool disposed in the wellbore. One type of downhole tool is an EM induction tool that may be used to make measurements of the electrical resistivity of earth formations penetrated by a wellbore or make measurements of distance and direction to a second well. EM induction tools may be used in logging-while-drilling/measuring-while-drilling operations, electromagnetic ranging, wireline logging, and permanent monitoring systems, among others. EM induction tools, or instruments, may typically comprise at least one transmitter and at least one receiver. The transmitter(s) and receiver(s) may be disposed on a tubular, such as a bottomhole assembly, mandrel, or casing joint. The EM induction tool may be implemented to determine the distance and direction to surrounding wells. Additionally, the EM induction tool may be disposed in a wellbore for the purpose of investigating electrical properties of subterranean formations and wells adjacent the wellbore. An electrical property of interest may be the electrical conductivity of particular portions of the formation. An alternating current having at least one frequency may be conducted through the transmitter(s). The alternating current may induce eddy current to flow within the surrounding subterranean formations or in adjacent well casings. This eddy current in turn may induce voltages in the receiver(s).
However, depending on the application, azimuthal currents could be flowing on the tubular associated with the EM induction tool. These azimuthal currents may constitute a significant portion of the direct signal at the receiver(s). The “direct signal” may be considered the signal recorded at the receiver(s) without any target present. The target may be a second wellbore, formation inhomogeneity, a bed boundary or an approaching water/carbon dioxide front. Thus, the direct signal would be present at the receiver(s) even in a homogenous formation. It is often desirable to minimize, reject, our process out the direct signal, as the direct signal may be very large compared to the target signal. Detecting the target signal in the presence of the direct signal often requires large dynamic range, which may be difficult to obtain in downhole electronics.
Currently, “gap sub” structures may be used for blocking axial currents on a downhole device. Gap sub structures may operate within galvanic application in which electrodes may create downhole device currents flowing primarily in the axial direction of the downhole device. However, there is a need to provide devices and methods for mitigating azimuthal current created within a downhole device by the transmitter. Characteristics of azimuthal current may be significantly different from axial currents that may be created by electrodes.